Biaxially-oriented, blown polyolefin films are generally known in the art and have been used in the production of articles such as garbage bags, shopping bags, food wraps, and any number of articles requiring polymer chain orientation in both the machine direction (MD) and the transverse direction (TD) of the film. Although cast films may be processed to achieve biaxial-orientation, blown films are generally preferred as they usually required less subsequent processing steps to achieve good mechanical properties, namely tensile strength, elastic modulus, and impact resistance, in both the machine and transverse directions.
It is also common practice within the manufacture of blown film resins to incorporate various stabilizers during the compounding and pelletization step of production. These stabilizers are usually antioxident compounds, namely phosphites or phenolics. Phosphites are generally used to prevent thermal degradation and other chemical interactions that may take place within the melt to negatively impact processing performance and mechanical properties of the resulting polymer film. Phenolics serve to protect the blown polymer film from the effects of aging or long term degradation that may occur over the lifetime of a film at ambient temperature conditions. Whether the polymer is degraded during processing or by aging, this will usually result in a loss of mechanical properties and significantly reduce the ultimate tensile strength and elongation at break percentage for the film. Accordingly, it is now common practice to add phosphite compounds to provide short term antioxident protection during manufacturing and phenolic compounds to provide long term antioxident protection against aging. Antioxident compounds are usually added in relatively small amounts and account for less than about 2% by weight of the resulting blown polymer film.
Polymer degradation in processing may occur in two forms, chain scission and cross-linking (also referred to as long chain branching when the cross-linking is moderate). In the extreme, such as polymer blends without antioxidants, both forms of degradation can be very detrimental to both the processablity of the melt and the physical properties of the blown film. Antioxidants, especially phosphites, protect the polymer and prevent degradation from occurring. However, it is notable that a polyethylene melt which has little long chain branching and narrow Theological breadth may tend to form an unstable film bubble which can be described as bubble breathing (vertical movement of the neck as a function of time), dancing (circular rotation of the bubble around the axis formed by the die center) or movement of the bubble relative to the die which is random in nature. These instabilities cause poor gauge distribution (i.e., variations in film thickness) and may also result in process interruptions that will generally lead to a reduction of the quantity of acceptable film that is produced. Accordingly, there is a need for an improved method of manufacturing blown polyethylene films which regulates the amount and ratios of antioxident additives used to improve film bubble stability and maximize blown film production rates. In addition, the optimization of the antioxident additive package in the resins can improve film mechanical properties. This is achieved with a controlled level of LCB which results in a better balance of machine and transverse direction orientation when processed on commercial blown film equipment at high throughputs.